HF surgical devices with an automatic start function exist, where the HF current is activated with the pre-set parameters as soon as there is sufficient contact between the instrument and the tissue. This is possible for both bipolar and monopolar applications. The device can be configured such that the HF current is activated automatically when the tissue is touched with forceps or a clamp and the impedance then falls below a pre-set impedance threshold (after a defined time lag has elapsed).
The systems known to date have two disadvantages with the above method: as a rule, the devices either have a separate sensor system for determining the impedance between the terminals, or the measurement technology of the HF generator, which normally serves to establish the current HF operating data, is used for this function in combination with an HF generator as a source of measuring current.
If the latter is the case, then the HF generator is continuously activated with minimal voltage and current values, is switched to the outputs and the HF signal is used as a measurement parameter for the impedance. The advantage of this technique is that the measurement technology of the HF generator typically enables very precise measurements, but the serious disadvantage is that the measurement signals have relatively large voltage amplitudes of some tens of volts as a result of the design. This results in a continuous potential interference with sensitive devices in the vicinity, such as ECG monitors.
Moreover, autostart functionality can only be enabled at a single output of the device, since if a plurality of outputs are arranged in parallel then it is not possible to determine which output contributes the decisive part of the total impedance. It is not possible to rapidly switch the signal between different outputs because of the relay to be switched.
If, however, a separate sensor system is used for the autostart functionality (see, for example, EP 1051948 B1), then the amplitude of the measurement signals can be kept substantially lower. However, in addition to the measurement technology of the HF generator, a further very precise sensor system is necessary for monitoring the impedance at the output. In principle, this can be realized at a plurality of outputs in parallel, allowing the function to be selected at a plurality of outputs at the same time. This is, however, expensive and not favorable economically due to the required accuracy of the sensor system.